1. Field of the Invention
The present invention relates generally to anamorphic lenses and more particularly to an anamorphic composite lens having a curved surface and a gradient index of refraction, a light beam scanning mechanism incorporating an anamorphic gradient-index lens and to a coupling lens for a multiply divergent and or axially astigmatic light source.
2. Description of the Prior Art
An article entitled "Laser diode to fiber coupling using anamorphic gradient-index lenses" authored by Joan M. Stagaman and Duncan T. Moore appeared in APPLIED OPTICS, Vol. 23, page 1730 on June 1, 1984. The article discusses a lens design approach using anamorphic gradient-index components to examine the problem of coupling light from a laser diode to a multimode fiber.
U.S. Pat. No. 4,025,157 which issued on May 24, 1977, to William E. Martin discloses a gradient-index miniature coupling lens. This patent is of interest in disclosing a semiconductor material for the miniature optical lens that is a composite material that is shaped in a configuration of pairs of parallel rectangular side surfaces, each such pair of surfaces being disposed orthogonally relative to each other pair. The composition of the semiconductor material varies between at least one pair of the parallel rectangular flat surfaces for causing a predetermined gradient of refraction which produces a desired focal length for light energy transmitted orthogonally to the gradient-index of refraction.
Laser scanning mechanisms in which a laser beam is focused onto a receptor page in a predefined position are well known in the art. With such a scanning mechanism, information can be written onto the page by scanning the laser beam across the page and alternately turning the beam on and off. The scanning of the laser beam across the page can be accomplished, for example, by a polygon scanner, a galvanometer mirror, a holographic element, or an acousto-optic modulator. Each scanning technique has its own advantages and disadvantages. However, because it is easily visualized, a rotating polygon scanner is illustrated in FIGS. 8 and 11. The disclosed prior art scanning mechanisms in FIGS. 8 and 11 are termed pre-objective scanning systems because the polygon scanners are placed before the objective scan lens. If the polygon scanners are placed after the objective scan lens (FIG. 23), the systems are referred to as post-objective scanning systems.
Independent of scanning technique, an error can result during the scanning operation which is called cross-scan error. This is best described as an undesirable displacement of the light beam perpendicular to the scan direction as the light beam is scanned across the page. This can cause a bowing of the line or undesirable shapes in the printed line. In the polygon scanner of FIG. 8, cross-scan error arising from angular error in a mirror facet of the scanner is depicted. The angular error or pyramid error in the facet misdirects the laser beam and causes the entire scan to be written on a vertically displaced scan line. Similarly, if the rotating scanner wobbles slightly about its rotational axis, a wobbling cross-scan error would result.
Another error resulting from the use of a cylindrical lens of the type shown in FIG. 9, when interposed between the polygon and objective scan lens, is referred to as a field curvature error. The cylindrical lens causes the laser beam to scan on an inward curving focal line rather than on a straight focal line. This field curvature error is rectified by the use of a known toroidal lens, as shown in FIG. 10. When such a lens is interposed between the polygon and objective scan lens, as seen in FIG. 11, a common but relatively expensive solution is provided for reducing cross-scan and field curvature errors.
The cylindrical lens in front of the polygon mirror of FIG. 11 brings the laser beam to a line focus on the mirror facet. The second toroidal lens recollimates the laser beam prior to the flat field objective scan lens. An angular error .alpha. in the polygon causes only a translational shift (no angular deviation) in the collimated laser beam between the toroidal lens and the objective scan lens. The objective scan lens maintains focus of the laser beam on the proper scan line.
Although the known toroidal lens shown in FIG. 11 offers a solution to the reduction of the cross-scan and field-curvature errors, the manufacturing of toroidal lenses is difficult and costly, primarily because for each surface two curvatures need to be fabricated.
This invention provides a solution to the problem of cross-scan and field-curvature errors that is of simple design and construction, and economical to manufacture. The solution involves replacing the more costly and difficult to manufacture toroidal lens by a less costly and easy to manufacture toroidal equivalent gradient-index lens, hereinafter referred to as TEGIL.